Chen Xiao Fan wrote:
> How much is the total budget power consumption? If it is not so high,
> quite some linear regulator will be able to cope with wide input
> voltage (say <=30V or even <=45V).

The bare circuit doesn't draw that much, but I wanted to provide reasonable
extra power for the custom part.  Yes, linear regulators can handle 30V, but
the power dissipation gets nasty.  Even with only 50mA at 5V, that's 1 1/4
watts dissipated by the regulator at 30V in.  That's more than just a 7805
sticking up is going to handle.  I know the linear regulator sounds so easy
and simple, but there are some messy issues once you look more closely.

> It also looks a bit strange
> for me to see MCP1700 here. Is it really safe to be used since its
> maximum voltage is only 6V (or 6V5)?

Yes, I've done this before in production designs.  Note that the switcher
produces a nominal 5.5V, so the MCP1700 is being run totally within specs.
The Microchip analog parts are very nice when you can live within the
constraints of 6V max.  When I did this in a production design, I looked at
various LDOs and the Mircrochip part had the best specs and either cost the
same or less - again considering the input would be 6V or less.

> I still like the idea of using a SMPS. As for simple SMPS controller,
> I think they are comparable to the price of 10F204 and saves you
> the cost of programming of 10F204. For example we have an old design
> using MC33063AD SO8 and it costs about US$0.3. It may even save you
> the external MOSFET Q5 and the associated driving circuits (R3, Q2, Q4,
> Q3, R2, R4 and C7)if your budget current consumption from 5.5V
> is less than 500mA (I think it should be the case since MCP1700
> can only deliver 250mA, unless you have other plan for the 5V5).

I looked around at this a while ago, and don't specifically remember the
switcher you mention.  In general, they were either too low input voltage,
too inefficient, or too expensive.  I've done the 10F trick a few times now,
and it works very nicely.  Note that the most expensive part in that circuit
is the inductor, which you have to supply yourself in any case.

And yes, I intend the 5.5V to be available beyond what the MCP1700 requires.
If more 5V is needed, additional LDOs can be added.  The 5.5V is also
available for things like relays that require significant current but can
tolerate some ripple.  At that input voltage, just about any 5V LDO can be
run at its maximum current without worrying about heat.  For example, the
MCP1700 will only dissipate 125mW at its maximum output of 250mA.


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